Monolithic bodies having longitudinal through passages are produced by forcing an extrudable plastic mixture of ceramic precursor materials through interconnecting die passages in which the material is shaped and coalesced into a structure of intersecting, relatively thin walls that define the passages. In one application the extrudable material is a mixture of materials, such as clay, talc and alumina, that can be fired to form cordierite or other heat resistant ceramic. Ceramic honeycomb-shaped monoliths formed of cordierite have been coated with suitable catalysts and catalyst support materials and used in the treatment of automobile exhaust gases. Ceramic honeycomb-shaped bodies are also being considered for use as particulate traps in diesel engine exhaust systems.
In one preferred method of extruding such monolith bodies a plate or disc extrusion die member is used. Feed holes for the extrudable material are provided in one face of the die and a network of intersecting slots are formed in the opposite face. The feed holes lead to and intersect the slots so that the material being extruded can be forced into the feed holes at the upstream side of the die and caused to flow into the die slots. The slots are arranged and sized so that the material spreads and coalesces therein into a unitary, relatively thin-walled honeycomb structure defined by the die slots. The extrudate emerges from the discharge side of the die as a long, unfired ("green") body. The body is cut into suitable lengths, dried and fired to produce a durable, monolithic honeycomb body.
Viewed on end, the body has a number of through passages defined by walls. The cross-section of the passages may be square, triangular, rectangular, or the like, depending on the slot arrangement in the extrusion die. Applications for such bodies have led to efforts to produce more, and thus smaller, passages per square inch of the body's cross-section. This has resulted in thinner walls and has required refinements in the technique of making the dies.
Briefly stated, the die can be produced by starting with a plate of clean, strong, machinable metal, such as, e.g., brass or steel. The plate must be sufficiently thick to withstand extrusion pressures even though it is perforated with many feed holes and discharge slots. The thin slots must be deep enough so the extrudable material flows laterally while flowing downstream. The lateral flow is necessary to completely fill out the walls of the extruded structure by the time the extrudate leaves the die. The extrudable material is delivered to the slots by means of many relatively small diameter feed holes drilled or otherwise suitably formed from the opposite side of the die plate. Each hole must precisely intersect a slot, forming a relatively unimpeded flow junction therewith. Usually the feed holes are located so that each terminates at a point of intersection of two slots.
Monolithic structures having 100 to 600 or more cells per square inch are now produced. The cell walls of such structures are quite thin, typically less than 0.010 inch. The intersections of the die slots are necessarily quite close together. As a result, the feed holes are small in diameter and closely spaced.
It has proven difficult to economically drill a large number of small feed holes located with sufficient accuracy to precisely intersect the extrusion slots in the die. Small diameter drill bits drift laterally as they penetrate the metal. Even a small displacement results in misalignment between a feed hole and a die slot. An exemplary die plate may be about one inch thick with feed holes about 3/4 inch deep. if the feed holes are located on 0.100 inch centers and must intersect a slot about 0.010 inch in width, it does not take much lateral movement of the drill bit in penetrating to a depth of 3/4 inch to partly miss the slot.